1. Field of the Invention
The present invention relates to broaching machinery in general and in particular to puller mechanisms for pulling a broach through a workpiece.
2. Brief Description of the Prior Art
The various types of known vertical pull-down broaching machines depend upon conventional automatic jaw-type puller mechanisms for their successful operation. Such conventional puller mechanisms utilize a series of circumferentially located puller jaws which are moved into and out of engagement with a circumferential groove located at the end of a normal broach tool. The jaws are controllably moved into and out of engagement with the groove by a cammed puller sleeve which is spring-loaded to move with respect to a stationary surface of the broaching machine such as the machine platen in response to movement of the broach into the puller mechanism. The circumferential groove on the broach end is usually made significantly wider than the puller mechanism jaws to allow consistent and positive engagement of the puller jaws with the groove.
During normal broaching operations the force of pulling the broach through the workpiece by the puller mechanism causes the engaged puller jaws to be forced against the top shoulder of the circumferential groove preventing any drop of the broach with respect to the puller jaws. This relationship of the puller jaws pressing against the top shoulder of the circumferential groove will be maintained as long as the pulling force on the broach exceeds the weight of the broach. However, when this stops being the case, the broach will drop vertically by the amount of the available clearance in the circumferential groove with respect to the puller jaws.
This condition occurs in every case of a broaching operation when the last broach cutting tooth has passed through the workpiece. For the majority of broaching jobs this inevitable drop of the broach tool presents no problem. However, there are cases when this dropping action is very detrimental to the success of a broaching operation.
One of such cases is where a heavy broach tool is subjected to a wide variation of broaching forces due to a particular desired design. A fairly common example is a broach taking heavy roughing cuts as it starts through a workpiece and then diminishing to very light finishing cuts during its travel through the workpiece. Depending on the weight of the broach tool in relation to the cutting force upon it, it is possible for the broach to drop while still engaged with the workpiece. Such a drop during engagement with the workpiece causes poor workpiece surface finish leaving chatter marks in the workpiece and erratic workpiece size variation. The machine also suffers due to erratic machine operation causing poor broach life and increasing machine maintenance due to vibration.
Another situation where such dropping during engagement of the broach with the workpiece is detrimental is found in the broaching of internal helical forms such as the helical annulus gears which are a part of every automotive automatic transmission. To broach a helical form the broach tool or workpiece must be rotated to provide the same lead and helix angle as the tool which is being pulled down linearly. Any dropping action of the broach can cause lead helix angle and/or involute form errors in addition to the previously mentioned undesirable factors.
In view of the foregoing, it will be seen that for certain broaching operations it is extremely desirable to have a jaw-type broach puller assembly which, while allowing a significantly larger circumferential groove at the end of the broach then the puller jaws, will prevent any dropping of the broach with respect to the puller jaws during any part of the broaching operation.